Manufacturing of integrated circuits is generally a procedure of forming thin films and layers of various materials on wafers of base semiconductor material, and selectively removing areas of such films to provide structures and circuitry. Doped silicon is a typical base wafer material. CVD is a well known process for depositing such thin films and layers. For example, polysilicon may be deposited from silane gas, SiH.sub.4. It is known, too, to deposit tungsten silicide from a mixture of gases including silane and a tungsten-bearing gas such as tungsten hexaflouride. Pure tungsten is also deposited on silicon wafers in the manufacture of integrated circuits, sometimes selectively and sometimes across the entire surface in a process known as "blanket" tungsten.
In a typical CVD process such as blanket tungsten, wafers are placed on supports within a sealable chamber, the chamber is sealed and evacuated, the wafers are heated, typically by heating the wafer support, and a gas mixture is introduced into the chamber. For example, in the blanket tungsten process, tungsten hexaflouride and hydrogen are introduced as reactive gases, and typically argon is introduced as a non-reactive carrier gas. The tungsten hexaflouride is the source of deposited tungsten. Typically the gases are flowed continuously during process. The temperature of a substrate (wafer) to be coated is one of the variables that drives the chemical reaction to cause tungsten to be deposited on the substrate surface. It is important to control the temperature, the concentration of various gases in the mixture introduced, and such characteristics as the uniformity of flow of gas over the surface being coated, among other variables.
An even thickness of a deposited layer is an important characteristic in CVD coating. One reason is that the process of making integrated circuits is generally a process of adding a layer of material and then removing portions of the added layer to provide structure on the wafer surface. Removal of portions of a layer is most often done by etching, which is a process of eroding material from the surface of a layer, usually in a specific pattern. It may be necessary, for example, to etch vias through an insulating layer to be able to subsequently provide electrical connection to structure beneath the insulating layer. If the insulating layer is not of uniform thickness, etching at a specific rate in one region on the wafer will open a via through the insulating layer before the same etch rate will accomplish the via in a region on the wafer where the insulating layer is thicker. There is no good way, though, to stop etching in the one region while continuing in another, so to finish the vias in the thicker regions will over-etch in the thinner regions, possibly damaging underlying structure in the thinner regions.
Another example of the importance of coating thickness uniformity is in the deposition of electrically conductive layers subsequently etched to provide electrically conductive interconnections between devices in an integrated circuit. If the coating thickness is not uniform, the cross section of the resulting interconnecting traces will vary, and the electrical conductivity of the traces will vary accordingly.
Wafer uniformity is typically measured and reported in percentages. A thickness uniformity of plus or minus 2% is a typical goal in CVD coating.
Much effort and expense is expended in CVD equipment design to promote thickness uniformity. For example, manifolding is provided to inject the mixture of coating gases into a chamber in a CVD process in a manner to promote uniformity, such as by injecting the coating gas mixture individually through a distribution apparatus at or near the surface of a wafer to be coated. It is known to the inventors, for example, to distribute the coating gas mixture through a "showerhead" device, which uses a perforated plate generally parallel to the surface of a wafer to be coated, and passes the gas mixture through holes in the plate arranged to provide the empirically "best" distribution, as measured by thickness measurement instruments after processing.
It has been found in practice that for gas distribution devices to work well, and to provide process stability and repeatability, it is typically very important that gases be thoroughly mixed before being introduced into the processing chamber. Even with such efforts and precautions, however, adequate uniformity is not always achieved. For example, it is commonly known and expected in the industry that thickness uniformity "drops off" near the outer periphery of a wafer. Virtually all profiles produced across a wafer diameter show this thickness degradation of the solid deposited film at the wafer's edge. There are various explanations in the art for the phenomenon, and many simulations have been done based on fluid dynamics theory that show the effect, as well as empirical studies.
The matter of gas composition and concentration at the point of process is no simple matter, because gases must be introduced at discrete points, and coating gas is "used up" at the point of process. Moreover, most CVD processes are performed at relatively low pressures. A total pressure in a process of chamber of less that 1 Torr is common, although some processes are now performed at pressures up to 30 Torr and higher.
CVD processing for such as semiconductor fabrication has been developed over a number of years, and there are many useful processes. In some simple processes, a single source gas, a source gas being a gas that contributes to the solid film formed, is decomposed by heat causing a material to deposit on a substrate, leaving gaseous products that are conducted away from the substrate. In such processes a carrier gas that does not participate in the chemical reaction is frequently used to help conduct the coating gas to the substrate.
In other processes, more than one gas introduced into the CVD chamber participates in a chemical reaction. In these instances, one gas may contribute all of the material that forms the solid coating, or a more complex coating may be formed by material contributed by more than one gas. An example of a process having more than one reactive gas in which both contribute to the coating is in the production of amorphous silicon nitride. In this process SiH.sub.4 and NH.sub.3 react to produce an amorphous solid film of silicon nitride generally represented by Si.sub.3 N.sub.4, and other products, notably hydrogen.
An example of a process in which there are two reactive gases, and one contributes all of the material for the solid film, is the reduction of tungsten hexaflouride by hydrogen to produce a tungsten or tungsten silicide film. Another is the reduction of tungsten hexaflouride by silane to produce a film.
Conventionally, as described in the background section above, all of the gases involved in a reaction are mixed prior to injection to the CVD chamber, and the proportions are controlled by careful measurement of flow through precision mass flow controllers. The precise proportions required are typically determined empirically, and are influenced greatly by the nature of the coating equipment. It is conventional wisdom also, to provide for uniform distribution of the coating gas mixture over a surface to be coated, and to provide for rapid replacement of depleted source gas at the point of process, that is, at the surface being coated. Even with the precautions described above, further improvement in thickness uniformity is always welcome, and provides monetary benefits in processing.
In addition to the problems that stem from uneven coating thickness, further problems may result from uneven etch rate in subsequent steps to pattern deposited film. An etch process that is not uniform over a wafer surface can have the same undesirable effect as a uniform etch on a non-uniform film. Moreover, problems can be further magnified if the film is non-uniform and the etch rate is also non-uniform.
What is needed is a greater degree of control over uniformity of coating and uniformity of etch rate, with an ability to alter uniformity profiles in a selective and controlled manner.